The present invention relates to a built-up structure of a rear fork for motorcycles, and more particularly to a built-up structure of a rear fork for motorcycles which is swingably supported at its front end portion from a body frame and supports at its rear portion a rear wheel axle.
A rear fork for motorcycles is constructed of a pair of left and right rear fork arms for supporting a rear wheel axle at their rear end portions, a cross member connecting the pair of left and right rear fork arms, and a pivot axle support member or members, and the rear fork is built-up either by welding the respective members made of steel pipes or steel sheets to each other or by welding Al-alloy extruded members, Al-alloy sheets or Al forged articles to each other. In these welded structures, the respective elements constituting the structure have an increased wall thickness in order to reinforce a lowered mechanical strength caused by welding, resulting in increase of a weight of a vehicle body and rise of a manufacturing cost. In addition, in the case of employing constituent elements made of Al-alloy, since the welding is difficult and lowering of a mechanical strength of the portions of the member material subjected to thermal influences is great, the above-mentioned tendency is more remarkable. Although a rear fork made by low-pressure casting has been commonly known as an example of use of Al-alloy material which is used contemplating reduction of a weight, the wall thickness of the product cannot be made as thin as in the case of practically worked products in view of the manufacturing process, and therefore, the effect of reducing a weight is little.